This invention relates to circuitry for coupling electronic signals between a computer and a plurality of magnetic tape transports, and more particularly to circuitry for automatically detecting the data density of a selected tape transport.
Magnetic tape is commonly used for storing digital data generated by, or to be read by, a digital computer. Tape transports, or decks, are commercially available from a variety of sources. These commercial units operate at various tape speeds up to and including 125 inches per second. Data density has been standardized to 800 and 1600 bytes per inch regardless of tape speed. At 800 bytes per inch, the NRZI, non-return to zero, method of recording data is used, while at 1600 bytes per inch the PE, phase encoded, method of recording is used. These data recording methods are defined by the American National Standards Institute, 1828 L Street, N.W., Washington, D.C. 20036. Each of these recording methods also requires its own electronic circuitry for encoding and decoding the recorded signals. Tape transports with electronics for either, or a combination of, the two recording methods are available. The manufacturers of tape transports also provide formatters for use with their transports.
A formatter is basically an electronic unit which converts digital data into the proper recording format, that is, NRZI or PE, and controls the recording of data on one or more transports. When a PE formatter, or a dual density formatter operating in PE density, starts recording on a new tape, it generates an ID, identification, burst. The ID burst consists of a series of alternate ones and zeros written on the tape starting before the BOT, beginning of tape, tab and ending past the tab. The presence of an ID burst therefore identifies a tape as a phase encoded tape. The BOT tab is typically a reflective tab sensed optically by the transport to provide a reference starting point. A formatter also typically generates a parity bit with each byte recorded to allow detection of recording errors and, additionally, generates special character, CRCC or LRCC, bytes for detecting errors within an entire record. A complete description of a formatter, designed for use with both NRZI and PE data, may be found in the Operating and Service Manual No. 101985, for Dual Formatter Models F6X8X and F8X8X, produced by Pertec Corporation, 9600 Irondale Avenue, Chatsworth, Calif. 91311.
Although commercially available formatters are compatible with their corresponding tape transports, they are not designed to be controlled by any particular central processing unit. Thus, for example, where it is desired to use a tape transport manufactured by Pertec Corporation in conjunction with a minicomputer manufactured by Interdata Corporation, an interface unit must be provided between the computing unit and the formatter. Some interface units are commercially available, but are typically limited to either NRZI or PE data formats. The few interface units which do operate with both data formats are very expensive and do not provide means for automatically determining the data density of a prerecorded tape placed on a tape transport for reading by the computing unit.
Accordingly, an object of the present invention is to provide an interface for coupling a computing unit of one manufacturer to a magnetic tape transport formatter of a second manufacturer.
Another object of the present invention is to provide a computing unit to tape transport formatter interface capable of coupling both NRZI and PE formatted data.
Another object of the present invention is to provide a computing unit to formatter interface which automatically selects the data format of a prerecorded tape and stores this format information.
Another object of the present invention is to provide a simple and inexpensive computing unit to formatter interface.
An interface according to the present invention comprises input and output buffer coupled to a computer multiplexer bus output, a command translator coupled to the computer output for converting computer commands into commands readable by a formatter, an automatic density selector for searching the beginning of a newly loaded tape for the presence of an identification burst and determining and storing the density of the tape from the presence or absence of such burst, and a status indication translator for converting status bits generated by said formatter into status indications readable by said computer. Substantially all of the circuitry used to form this interface is standard integrated circuit logic cells.